1. Field of the Invention
Embodiments of the invention described herein pertain to the field of shipboard transportation of liquefied natural gas (“LNG”). More particularly, but not by way of limitation, one or more embodiments of the invention describe systems and methods of gas-up and cool down of LNG cargo tanks located in a waterway location.
2. Description of the Related Art
Natural gas is often carried onboard special cryogenic tanker ships from the location of its origin to the location of consumption. In this way, natural gas may be transported to areas with a higher demand for natural gas. Since LNG occupies only about 1/600th of the volume that the same amount of natural gas does in its gaseous state, liquefying the natural gas for transport facilitates the transportation process and improves the economics of the system. LNG is produced in onshore liquefaction plants by cooling natural gas below its boiling point (−259° F. (−162° C.) at ambient pressures). The LNG may be stored in cryogenic cargo tanks located on special cryogenic tanker ships, either at or slightly above atmospheric pressure. Typically, the LNG will be regasified prior to its distribution to end users.
In a conventional cryogenic cargo cycle, tanks on a cryogenic tanker ship are full of fresh air which allows maintenance on the tank and pumps. For example, the tanks are full of fresh air when the cryogenic tanker ship comes out of the yard, after dry docking or repairs, if the ship has been sitting idle, or has burned off all of the remaining natural gas in the take (for example, burning off a heel). The cryogenic cargo cannot be loaded directly into the tanks until the fresh air (for example, oxygen) is replaced with an inert gas to inhibit explosions within the tanks. The tanks may be filled with inert gas (for example, carbon dioxide) until the atmosphere in the tanks contains less than 4% oxygen. Carbon dioxide, however, freezes at temperatures used to store liquefied natural gas, thus the carbon dioxide must be removed prior to filling the tanks with liquefied natural gas. To remove the carbon dioxide from the tanks and the tanks conditioned to receive a cold fluid, the tanks under go a gas-up and cool down procedure.
The cryogenic tank ship is docked at a port and connected to a gas-up and cool down system that includes cryogenic loading arms (hard arms) and/or rigid pipe suitable for handling cryogenic fluids. During gas-up, the inert gas atmosphere in the cargo tanks and piping systems of the cryogenic tanker ship is displaced with natural gas. Next, the cargo tanks are cooled down by slowly reducing the temperature of the cargo tank atmosphere and surrounding containment to temperatures of about −140° C. Once the cargo tanks are cooled, LNG may be loaded into the cargo tanks without subjecting the tanks to cold shock. The gas-up and cool down operation takes approximately 34 to 72 hours before the LNG cargo may be loaded onto the cryogenic tank ship.
During the gas-up and cool down operation, the portion of the dock involved in the operation is not available for shipping operation (for example, unloading and loading LNG, and/or the use of liquefaction trains) and/or terminal access is limited. Thus, there is a need for more efficient systems and methods for treating of LNG cargo tanks.